How to check for the existence of a module without an error being raised?

Question

In Angular 1.2, ngRoute is a separate module so you can use other community routers like ui.router instead.

I'm writing an open-source module that aims to work for multiple different router implementations. So how can I check which router is loaded or exists?

I'm doing the following inside a factory in my module, but it does not work the way I expect it to:

if (angular.module("ngRoute"))
  // Do ngRoute-specific stuff.
else if (angular.module("ui.router"))
  // Do ui.router-specific stuff.

It raises an error for whichever module is not loaded. For example, if the app is using ui.router, then the following error is raised for the ngRoute check:

Uncaught Error: [$injector:nomod] Module 'ngRoute' is not available! You either misspelled the module name or forgot to load it. If registering a module ensure that you specify the dependencies as the second argument.

Answer 1

I am not aware of a way of checking without an error being raised; however, notice that the issue is that it was an Uncaught Error, not that an error was thrown. The pattern for catching such an error is the following.

try { angular.module("ngRoute") } catch(err) { /* failed to require */ }

If an error is caught, you can try the other module, and if not, you can use the first.

If your behavior will be the same for each module, you could do something like the following, in which we define a function which will attempt the first of the listed module names, and if an error is thrown, try the next option.

var tryModules = function(names) {
  // accepts a list of module names and
  // attempts to load them, in order.

  // if no options remain, throw an error.
  if( names.length == 0 ) {
    throw new Error("None of the modules could be loaded.");
  }

  // attempt to load the module into m
  var m;
  try {
    m = angular.module(names[0])
  } catch(err) {
    m = null;
  }

  // if it could not be loaded, try the rest of
  // the options. if it was, return it.
  if( m == null ) return tryModules(names.slice(1));
  else return m;
};

tryModules(["ngRoute", "ui.router"]);